As a valve mechanism for opening and closing an intake or exhaust valve of an engine, there has been proposed a camshaft including a tubular cam tube including one or more cam parts, and a driving shaft inserted into the cam tube (for example, see PTL 1).
In the valve mechanism, the driving shaft which transmits rotary power transmitted from a power transmission device rotates, and the cam tube integrally rotates with the driving shaft via spline fitting, so that the intake or exhaust valve is opened and closed. A switching device is configured to move the cam tube in an axial direction of the driving shaft, so as to switch a plurality of cam profiles provided on the cam part with respect to one intake or exhaust valve.
A male spline formed on an outer tubular surface of the driving shaft can be machined by cutting machining or rolling machining regardless of a spline groove length. A female spline formed on an inner tubular surface of the cam tube can be machined by broaching machining or electric discharge machining.
However, in the broaching machining, since a cutting tool enters into an inner diameter, a cutting load during the machining is received by a thin shaft, and therefore, the spline groove length is limited to one to two times of a spline nominal diameter. Also, only one female spline can be formed in one cam tube. That is, although there is no problem for a cam tube extending over one or two cylinders, for a cam tube extending over three or more cylinders, a female spline groove length becomes short.
In this case, for the cam tube extending over three or more cylinders, when the female spline groove length is shortened, a driving torque for opening and closing the intake or exhaust valve cannot be transmitted sufficiently via the spline fitting from the driving shaft.
On the other hand, even if the intake or exhaust valve can be opened and closed with a small driving torque by reducing a biasing force of a valve spring of the intake or exhaust valve, as the biasing force of the valve spring decreases, the intake or exhaust valve may be stuck or opened due to exhaust pressure, and reliability of the engine may be impaired.
Therefore, in the cam tube extending over three or more cylinders, countermeasures are taken such as increasing the spline nominal diameter, or providing a plurality of cam tubes for one camshaft.
However, when the spline nominal diameter is increased to secure a sufficient spline groove length, a diameter of the cam tube is increased and the camshaft becomes heavy and large, so that mountability of the camshaft to the engine deteriorates. Further, when a long cam tube only includes one female spline, depending on a position where the female spline is arranged, the cam tube may be inclined when the cam tube is moved in the axial direction of the driving shaft. Further, when a plurality of cam tubes are provided for one camshaft, switching devices for switching the cam profiles is required by a number of division, and the valve mechanism becomes heavy and large, so that mountability of the valve mechanism to the engine deteriorates.
On the other hand, in the electric discharge machining, it is difficult to ensure machining accuracy when the spline groove length is long. Also, a device for performing the electric discharge machining becomes large in scale, manufacturing cost is significantly increased, and mass production becomes difficult.